Zdenek Hubicka

TiO2 and Fe2O3 Films for Photoelectrochemical Water Splitting

Titanium oxide (TiO2) and iron oxide (α-Fe2O3) hematite films have potential applications as photoanodes in electrochemical water splitting. In the present work TiO2 and α-Fe2O3 thin films were prepared by two methods, e.g., sol-gel and High Power Impulse Magnetron Sputtering (HiPIMS) and judged on the basis of physical properties such as crystalline structure and surface topography and functional properties such as simulated photoelectrochemical (PEC) water splitting conditions. It was revealed that the HiPIMS method already provides crystalline structures of anatase TiO2 and hematite Fe2O3 during the deposition, whereas to finalize the sol-gel route the as-deposited films must always be annealed to obtain the crystalline phase. Regarding the PEC activity, both TiO2 films show similar photocurrent density, but only when illuminated by UV light. A different situation was observed for hematite films where plasmatic films showed a tenfold enhancement of the stable photocurrent density over the sol-gel hematite films for both UV and visible irradiation. The superior properties of plasmatic films could be explained by ability to address some of the hematite drawbacks by the deposition of very thin films (25 nm) consisting of small densely packed particles and by doping with Sn.

Photoanodes with Fully Controllable Texture: The Enhanced Water Splitting Efficiency of Thin Hematite Films Exhibiting Solely (110) Crystal Orientation.

Hematite, α-Fe2O3, is considered as one of the most promising materials for sustainable hydrogen production via photoelectrochemical water splitting with a theoretical solar-to-hydrogen efficiency of 17%. However, the poor electrical conductivity of hematite is a substantial limitation reducing its efficiency in real experimental conditions. Despite of computing models suggesting that the electrical conductivity is extremely anisotropic, revealing up to 4 orders of magnitude higher electron transport with conduction along the (110) hematite crystal plane, synthetic approaches allowing the sole growth in that direction have not been reported yet. Here, we present a strategy for controlling the crystal orientation of very thin hematite films by adjusting energy of ion flux during advanced pulsed reactive magnetron sputtering technique. The texture and effect of the deposition mode on the film properties were monitored by XRD, conversion electron Mössbauer spectroscopy, XPS, SEM, AFM, PEC water splitting, IPCE, transient photocurrent measurements, and Mott-Schottky analysis. The precise control of the synthetic conditions allowed to fabricate hematite photoanodes exhibiting fully textured structures along (110) and (104) crystal planes with huge differences in photocurrents of 0.65 and 0.02 mA cm(-2) (both at 1.55 V versus RHE), respectively. The photocurrent registered for fully textured (110) film is among record values reported for thin planar films. Moreover, the developed fine-tuning of crystal orientation having a huge impact on the photoefficiency would induce further improvement of thin hematite films mainly if cation doping will be combined with the controllable texture.

Effect of nitrogen doping on TiOxNy thin film formation at reactive high-power pulsed magnetron sputtering

The paper is focused on a study of formation of TiOxNy thin films prepared by pulsed magnetron sputtering of metallic Ti target. Oxygen and nitrogen were delivered into the discharge in the form of reactive gases O2 and N2. The films were deposited by high-power impulse magnetron sputtering working with discharge repetition frequency f = 250u2009Hz at low (p = 0.75u2009Pa) and high (p = 10u2009Pa) pressure. The substrates were on floating potential and thermally stabilized at room temperature during the deposition process. Post-deposition thermal annealing was not employed. The chemical composition from x-ray photoelectron spectroscopy diagnostic reveals formation of TiOxNy structure at low flow rate of oxygen in the discharge gas mixture. This result is confirmed by XRD investigation of N elements incorporation into the Ti–O lattice. Decrease in band-gap to values Eg ~ 1.6u2009eV in TiOxNy thin film is attributed to formed Ti–N bonds. The discharge properties were investigated by time-resolved optical emission spectroscopy. Time evolution of particular spectral lines (Ar+, Ti+, Ti) is presented together with peak discharge current.

Time-resolved investigation of dual high power impulse magnetron sputtering with closed magnetic field during deposition of Ti–Cu thin films

Time-resolved comparative study of dual magnetron sputtering (dual-MS) and dual high power impulse magnetron sputtering (dual-HiPIMS) systems arranged with closed magnetic field is presented. The dual-MS system was operated with a repetition frequency 4.65 kHz (duty cycle ≈50%). The frequency during dual-HiPIMS is lower as well as its duty cycle (f=100u2002Hz, duty 1%). Different metallic targets (Ti, Cu) and different cathode voltages were applied to get required stoichiometry of Ti–Cu thin films. The plasma parameters of the interspace between magnetrons in the substrate position were investigated by time-resolved optical emission spectroscopy, Langmuir probe technique, and measurement of ion fluxes to the substrate. It is shown that plasma density as well as ion flux is higher about two orders of magnitude in dual-HiPIMS system. This fact is partially caused by low diffusion of ionized sputtered particles (Ti+,Cu+) which creates a preionized medium.

Deposition of electronic quality amorphous silicon, a-Si:H, thin films by a hollow cathode plasma-jet reactive sputtering system

High quality hydrogenated amorphous silicon, a-Si:H, thin films were deposited by means of a dc hollow cathode plasma-jet with magnetic field confinement. Single-crystal silicon nozzles were reactively sputtered in a high density hollow cathode discharge. Only nontoxic gases, argon and hydrogen, were used for this purpose. Different configurations of the dc hollow cathode were used for the deposition process. Electronic quality a-Si:H thin films were achieved with light to dark conductivity ratios >106, with light conductivity near 10−5 S/cm and dark conductivity between 10−11 and 10−12 S/cm. This was accomplished with a specific configuration of the hollow cathode discharge in the silicon nozzle. Our best films have a Tauc band gap near 1.8 eV and an atomic hydrogen concentration of about 14%. The growth rate achieved for the electronic quality a-Si:H films was in the range of 2–3 μm/h.

Spectroscopic ellipsometry applied to phase transitions in solids: possibilities and limitations

The possibilities of in situ spectroscopic ellipsometry applied to phase transitions investigation in oxide thin films and crystals are examined in this work, along with the use of various parameters calculated from ellipsometric data (band gap energy Eg, refractive index n and surface roughness) together with the directly measured main ellipsometric angles psi and Delta, for the detection of phase transitions. The efficiency of spectroscopic ellipsometry on surface phase transition and its sensitivity to surface defects are also demonstrated.

Plasma diagnostics of low pressure high power impulse magnetron sputtering assisted by electron cyclotron wave resonance plasma

This paper reports on an investigation of the hybrid pulsed sputtering source based on the combination of electron cyclotron wave resonance (ECWR) inductively coupled plasma and high power impulse magnetron sputtering (HiPIMS) of a Ti target. The plasma source, operated in an Ar atmosphere at a very low pressure of 0.03u2009Pa, provides plasma where the major fraction of sputtered particles is ionized. It was found that ECWR assistance increases the electron temperature during the HiPIMS pulse. The discharge current and electron density can achieve their stable maximum 10 μs after the onset of the HiPIMS pulse. Further, a high concentration of double charged Ti++ with energies of up to 160u2009eV was detected. All of these facts were verified experimentally by time-resolved emission spectroscopy, retarding field analyzer measurement, Langmuir probe, and energy-resolved mass spectrometry.

Investigation of ionized metal flux in enhanced high power impulse magnetron sputtering discharges

The metal ionized flux fraction and production of double charged metal ions Me2+ of different materials (Al, Cu, Fe, Ti) by High Power Impulse Magnetron Sputtering (HiPIMS) operated with and without a pre-ionization assistance is compared in the paper. The Electron Cyclotron Wave Resonance (ECWR) discharge was employed as the pre-ionization agent providing a seed of charge in the idle time of HiPIMS pulses. A modified grid-free biased quartz crystal microbalance was used to estimate the metal ionized flux fraction ξ. The energy-resolved mass spectrometry served as a complementary method to distinguish particular ion contributions to the total ionized flux onto the substrate. The ratio between densities of doubly Me2+ and singly Me+ charged metal ions was determined. It is shown that ECWR assistance enhances Me2+ production with respect of absorbed rf-power. The ECWR discharge also increases the metal ionized flux fraction of about 30% especially in the region of lower pressures. Further, the suppression o...

DEPOSITION OF NANOCRYSTALLINE AND MICROCRYSTALLINE BaXSr1-XTiO3 BY MEANS OF PULSE MODULATED LOW PRESSURE PLASMA JET SYSTEM

ABSTRACT RF plasma jet system was used for deposition of BaxSr1-xTiO3 (BSTO) and SrTiO3 (STO) thin films. Optical emission spectroscopy was used for control of concentration of particles sputtered from the hollow cathode. Relation between ratio of spectral intensity of Ba, Ba+, Sr and Sr+ lines and ratio of Ba and Sr concentration in the deposited film was found. Deposited thin films were analyzed by XRD, which confirmed presence of BSTO perovskite phase in the films. All prepared films with high enough Ba concentration (0.5 ≤ x ≤ 0.9) revealed good shaped dielectric hysteresis loops typical for ferroelectric phase.

Study of mass and cluster flux in a pulsed gas system with enhanced nanoparticle aggregation

The paper is focused on investigation of enhanced metal (Cu) cluster growth in a source of Haberlands type using pulsed gas aggregation. The aggregation Ar gas was delivered into the cluster source in a pulse regime, which results in the formation of well pronounced aggregation pressure peaks. The pressure peaks were varied by varying the different pulse gas frequency at the same mean pressure kept for all experiments. Hence, we were able to study the effect of enhanced aggregation pressure on cluster formation. Time-resolved measurements of cluster mass distribution were performed to estimate the mass and particle flux. The paper demonstrates that pulse gas aggregation influences growth of Cu nanoparticles, i.e., cluster mass/size, mass flux, and particle flux emitted from the cluster source. It was found that cluster mass related quantities are strongly influenced by pulsed gas frequency; the highest value of mass flux appears at the most pronounced pressure peaks. On the other hand, the particle flux depends only slightly on the gas pulse frequency. The explanation based on cooling and thermalization of sputtered particles is discussed in the paper.